The effects of single oral doses of 3,3' , 4,4', 5,5'-; 2,2', 4,4', 5,5'-; and 2,2', 3,3', 5,5'-hexachlorobiphenyls on lipid metabolism and cell structure, as well as on induction of various enzyme activities in the livers of rats have been studied. Structure-dependent effects have been observed, and effects of 2,2', 3,3', 5,5'-hexachlorobiphenyl demonstrate that p-substitution is not essential for biological activity.